FIELD OF THE INVENTION
The invention relates to a controller for controlling a reactive series compensator serially inserted at compensator terminals into a power transmission line. Typically, in such a transformerless reactive series compensator an inverter control is performed in order to control the line current and/or the voltage applied from the compensator to the transmission line. The voltage/current control enables control of the flow of power from one end to the other end of the transmission line and a power flow into the inverter of the compensator in order to charge a capacitor which provides a compensator terminal output voltage at the compensator terminals.
Typically, as will be explained below with more details, a current feedback control loop and a voltage feedback control loop are employed in order to respectively control the reactive and the active part of the line current. For doing so, the modulation signal based on which the PWM control of the inverter is performed, is a sinusoidal signal of a particular phase. Adjusting the amplitude and the phase of the modulation signal allows the power control.
Using such control loops, the output voltage of the compensator can be controlled by a modulation index with a constant DC voltage. Alternatively, the output voltage can be controlled by the DC voltage only using a constant modulation index, as the paper by A. Beer, H. Stemmler, H. Okayama, "A Hybrid Transformerless Reactive Series Compensators", EPE 1999--Lausanne describes (see page 8 of this paper). However, using constant DC voltage, the injection of harmonics into the transmission line by the inverter cannot be reduced even if the fundamental component is smaller than the DC voltage. Furthermore, using constant modulation index control, the controllability of the DC voltage is degraded in zero or very low DC voltage region since the active voltage cannot be sufficiently injected into the transmission line. Therefore, this method is not applicable to a full range operation. Furthermore, with the aforementioned two different control methods it is also difficult to reduce DC voltage dependent losses such as the switching loss of switching elements of the inverter and leakage loss of the DC capacitor of the compensator.
The present invention in particular addresses the problem as to how the injection of harmonics by a compensator controlled by a controller including a current and voltage control loop can be reduced. The present invention also addresses the other aforementioned problems.
Hereinafter, first the general background of transformerless reactive series compensators and of controllers for controlling a compensator and comprising a current and voltage control loop will be described.